This invention relates to liquid developing devices for use in electrophotographic copying machines, which devices are equipped with a plurality of paired electrode rollers, and more particularly to a developing device, in which a plurality of paired conductive rollers are positioned in a liquid developer consisting of an organic solvent (insulating liquid) and charged-toner particles which are dispersed therein, with the aforesaid paired conductive rollers being electrically connected, for example, by being grounded, to function as developing electrodes, whereby an electrostatic latent image carried on a copying sheet may be developed, during the time which the copying sheet is being transported by the aforesaid paired conductive rollers.
A developing device of the aforementioned type provides an electrode-to-electrode spacing smaller than that of a developing device equipped with electrode plates, and presents enhanced functions of the developing electrodes, so that a solid pattern may not only be reproduced in a satisfactory manner, but also there may be achieved reproduction of an electrostatic latent image at a potential of at least as low as 100V, as in the case of an electrostatic latent image on a copying sheet for use in an electrostatic latent image transfer process. The developing device of this type has been applied in a commercial electrostatic latent image transfer type copying machine.
The inventors discovered that a developing device of the aforementioned type suffers from the following shortcomings:
In the case where an electrostatic image on a copying sheet P provides distinct boundaries between a portion having charges and a portion free of charges, as shown in FIGS. 2A and 2B, in which there are shown solid patterns having charges (negative charges in the drawings) over the entire areas defined within a rectangle A1 and a triangle A2, respectively, and even though the copying sheet P itself is sufficiently dried, there develops black fogging in the form of lines, and whitened blanks. In addition, the inventors discovered that even in the case of the aforesaid electrostatic image, there develops no such a phenomenon, when the copying sheet itself is wet.
Moreover, in the case where an electrostatic image on the copying sheet is a solid pattern consisting of an inverted triangle A3 as shown in FIG. 2C, then no such a phenomenon as described above develops.
In the case where the electrostatic images provide a plurality of solid patterns of a rectangular shape, having a small width and arranged in parallel, and including a plurality of line patterns, as shown in FIG. 2D, then the aforesaid phenomenon also occurs. In other words, in those cases where the total width of the solid patterns is considerable, as measured at their rear ends in the direction that the copying sheet is being fed and also where the electrostatic images have distinct boundaries between charged portions and non-charged portions, and the copying sheet itself is sufficiently dried, then the fogging and whitened blank phenomenon occurs on the copying sheet.
The following is a more detailed description of the aforesaid fogging and whitened blank phenomenon taken in conjunction with an electrostatic image in the form of a solid rectangular pattern as shown in FIG. 2A.
FIG. 3 shows the results when the copying sheet carrying the aforesaid solid pattern was developed. In other words, a character A represents a normal toner image portion corresponding to an electrostatic image A, while characters B, F represent fogging in the form of black lines, and characters E, C, D represent whitened blanks.
The aforesaid fogging and whitened blank phenomenon is attributable to the fact that induced charges develop on the conductive developing rollers due to electrostatic images on the copying sheet. This will be described in more detail in conjunction with FIGS. 4A, 4B. FIG. 4A illustrates a whitened blank in portions E of FIG. 3 for the condition where a solid pattern on the copying sheet P is positioned between conductive rollers 3 and 4 which are both grounded. Copying sheet P consists of conductive substrate 2 and dielectric layer 1 coated thereon. Carried on dielectric layer 1 is a negatively charged solid pattern latent image.
Positive charges having a polarity opposite to that of the electrostatic image are induced onto the surface of conductive roller 3, which is positioned right above the solid pattern, while negative charges having the same polarity as that of the electrostatic image are induced onto those portions of the surface of the roller 3, which correspond to the opposite side portions E of the copying sheet P.
The reason why negative charges are induced onto such portions of the surface of the roller 3, which correspond to the opposite side portions E, is that part of the positive charges present on the conductive substrate 2 in correspondence with the electrostatic image on the dielectric layer 1 on the copying sheet P are freed due to neutralization of the electrostatic image by toner particles (positively charged) upon development, with the result that the charges thus freed are shifted to such portions of the surface of the roller 3, which correspond to the opposite side portions E on the conductive substrate 2 and remain there.
FIG. 4A shows the distribution of charges induced onto copying sheet P and conductive rollers 3, 4, in which there are produced electric fields directed from the positive charges to the negative charges as shown by the arrows, so that toner particles having positive charges are shifted in the same direction as that of the electric fields. The shifted positive charges then cling to electrostatic image A having negative charges, while the toner particles are repelled to the opposite side portions E. This results in a whitened blank phenomenon in portions E.
FIG. 4B illustrates the fogging and whitened blank phenomenon in portions B, C of FIG. 3 and shows a condition immediately after the rear end portion of the solid pattern has passed through conductive rollers 3, 4, i.e., a condition where portions B, C in FIG. 3 are positioned between the conductive rollers. When the solid pattern passes through the conductive rollers, then the polarity of charges on the surface of conductive roller 3 is abruptly reversed, as shown.
The reason why negative charges are induced onto such a portion of the surface of the roller 3, which corresponds to portion C, is considered to be a combination of normal induction resulting when, as in the case of the aforesaid portions E, part of the positive charges present on conductive substrate 2 of copying sheet P are shifted, and also because of instantaneous induction resulting from a change in the electric field due to the abrupt shifting of the charges in the solid pattern, which shifting results from the copying sheet being transported by the conductive rollers. It can be seen from the foregoing that the whitened blank phenomenon in portion C is more remarkable, as compared with that in portions E.
The reason why positive charges are induced onto the portion of the surface of the roller 3, which corresponds to portion B, is considered to be a combination of normal induction resulting from the induction of negative charges onto portion B in substrate 2, due to the shifting of part of the positive charges present on conductive substrate 2 to the portions E and C on substrate 2, and also because of instantaneous induction resulting from a change in the electric fields produced due to the abrupt shifting of induced charges onto the surface of conductive roller 3. Accordingly, there develops an electric field, as shown, in portion B, so that toner particles having positive charges cling thereto, thus resulting in fogging.
A strip type whitened blank phenomenon in portion D of FIG. 3 is considered to take place immediately after the solid pattern passes through the rollers. The reason for this is that a great amount of toner particles cling to the solid pattern, and thus there results a temporary lack of toner due to insufficient diffusion thereof.
The fogging phenomenon in portion F of FIG. 3 is shown in the form of a strip, in contrast to that of portion B, but presents low density fogging. This may be attributed to the fact that toner particles clinging to the surface of conductive roller 3 upon occurrence of the whitened blank phenomenon in portion E are off-set.
With the electrophotographic liquid developing device of such an arrangement wherein a plurality of paired electrode rollers are each electrically connected, for example by being grounded, in the case where the copying sheet is sufficiently dried, and the electrostatic image on the copying sheet is a solid pattern and the rear end portion or portions of the pattern have considerable width or total widths, there necessarily results a fogging and whitened blank phenomenon.